This invention relates, in general, to methods and apparatuses for determining the life expenditure of certain component parts of fluid driven turbomachines; and, in particular, the present invention relates to a method and apparatus for determining the amount of life expended in a turbomachine rotor which is subject to cyclic duty.
The long-term reliability and availability of turbines requires close attention to, and recognition of, the impact of thermal cycles on the "life" of steam turbine components. Excessive thermal transients imposed on turbine rotating and stationary parts during startups, major load changes, and shutdowns can greatly reduce the life of these components. Because the effect of each thermal cycle is usually in itself small, the effects may not be readily apparent to plant operators during the event and may not be recognized and properly considered in making day-to-day operating decisions. However, the long-term cumulative effects of such cycles can result in extended and expensive forced outages requiring major repairs or component replacement.
Prior to this invention, the expenditure of rotor life due to low cycle fatigue caused by thermal stress was regulated by assigning a target for each startup - shutdown cycle. An attempt would be made to control nominal surface stress during the startup, by temperature ramp or computerized stress control, so that the peak stress value would be less than half the allowable stress range for the target life expenditure. This approach erroneously assumes symmetrical startup - shutdown stress cycles and ignores cycles caused by boiler excursions, changes in steam admission modes (full arc to partial arc), or large changes in steady state load. It also requires a level of planning and record keeping which may not be feasible in many plants.
The present invention monitors turbine operating conditions to obtain rotor surface stresses, calculates a stress range for each cycle and outputs the equivalent percent life expended to a non-volatile incremental memory and display. The output information will be valuable in planning plant load distribution and will prove useful in operation by highlighting operating practices or problems which cause large cyclic life expenditures.
U.S. Pat. No. 3,950,985 to Buchwald and Busse is one example of one prior art practice. In that patent, equivalent hours of operating time for a thermal power plant are determined by actual operating stresses dependent upon operating temperatures. The apparatus of the patented invention sums a calculated stress based on actual turbine temperature with a curve representative of constant service life at that temperature. If the output of the summing junction indicates a higher stress than is permitted under constant service life conditions then an appropriate switching device weights the output of a pulse generator to output an equivalent higher service life used. A digital device counts the equivalent hours of service life expended. While the patented invention is useful in determining component life expenditure due to creep at varying operating temperatures, it fails to recognize low cyclic fatigue duties wherein the resultant stress may be either positive or negative depending on whether the component is a heating or cooling portion of the cycle.